Add the --pypi option to the show command

This pull request extends the show command with functionality for getting package information from PyPi. This was was originally proposed in #485. Usage is follows:

```
$ pip show --pypi ipdb evdev
---
Name: ipdb
Version: 0.7
Summary: IPython-enabled pdb
Author: Godefroid Chapelle
Url: http://pypi.python.org/pypi/ipdb
Homepage: https://github.com/gotcha/ipdb    
---
Name: evdev
Version: 0.3.1
Summary: bindings for the linux input handling subsystem
Author: Georgi Valkov
Url: http://pypi.python.org/pypi/evdev
Homepage: https://github.com/gvalkov/python-evdev
```

I've also made some small changes to the `pip show`'s exit status:

```
$ pip show --pypi ipdb evdev &>/dev/null ; echo $?
0
pip show --pypi non-existant &>/dev/null ; echo $?
23  # status.NO_MATCHES_FOUND
pip show --pypi ipdb non-existant &>/dev/null ; echo $?
0
# same logic applies to pip show ...
```

I couldn't figure out the how to use the mock xmlrcp transport, so I haven't written any unit tests yet.